It is well known to provide an aircraft slat assembly comprising a plurality of slats located adjacent to one another. The slats are typically moveable (by an actuator mechanism) between an extended position in which the slats are simultaneously deployed, and a retracted position in which the slats are simultaneously stowed on the fixed wing leading edge.
The actuator mechanism for this type of application tends to be extremely robust. In a known design, the actuator mechanism has met all the airworthiness requirements to date, however new airworthiness requirements may now require failure of the actuator mechanism to be considered. In the event of this theoretical failure of the actuator mechanism, differential drive of the slats could occur. This may, for example, lead to skew or misalignment, of one or more of the slats. In theory a failure in the actuator mechanism may also affect (for example, it may isolate) an associated detection and/or protection system; this increases the risk of excessive skew or misalignment occurring.
Detection systems to detect the magnitude of misalignment of slats and to shut down the slat actuator mechanism in the event of excessive misalignment, have been suggested. For example, EP0726201 describes a cable arrangement, in which the displacement of the cable is used to detect when a slat becomes skewed beyond normal limits. The system stops further movement of the slats, once excessive skew has been detected. By way of another example, EP 1088753 describes a skew detection system in which a control computer is arranged to shut down operation of a slat if the slat becomes skewed by more than a predetermined value. The amount of skew is measured by elements on the end of adjacent slats, which slide over one another when a slat becomes skewed, thereby reducing the effective length of a cable. These detection systems can be complex. The above-mentioned systems also rely on a significant degree of skew occurring before further movement of the slats is stopped, which may risk damage to the slat assembly.